The decline from its position at No. 20 in 2009 has been spurred by the Obama administration's aggressive prosecution of whistleblowers, said Delphine Halgand, the organization's U.S. director.

"We consider that the Obama administration has launched a war against whistleblowers," Halgand said. "This year is a continuation of the concern we already expressed that national security protection has been more and more threatening freedom of information in the U.S."

The U.S. has prosecuted eight alleged whistleblowers under the Espionage Act, more than all previous presidential administrations combined, Halgand said.

Read More

Despite Obama's campaign pledge to make his administration one of the most transparent in history, reporters and press freedom watchdogs have continually slammed the administration as one of the least transparent and criticized its dogged efforts to plug leaks.

Reporters without Borders, an organization that works to protect journalists around the world, tied the U.S.'s drop in this year's rankings to the "judicial harassment" of James Risen, a New York Times reporter who last year stared down federal prosecutors pushing him to reveal the identity of an anonymous source.

The purported source, ex-CIA officer Jeffrey Sterling, was convicted early this year on nine felony counts for leaking classified information. Risen refused to reveal his source, and federal prosecutors decided not to indict him on any charges.

Reporters without Borders also pointed to the Ferguson protests, where it says at least 15 journalists were arrested.

Fellow Western democracies scored better than the U.S. in the ranking.

Finland snagged the top spot, Canada was ranked 8th, and other U.S. allies, the United Kingdom and France, came in 34th and 38th, respectively.

France and the U.K. have also slipped in the rankings in recent years over concerns that national security concerns are encroaching on press access, Halgand said.

Other countries that got higher marks than the U.S. for press freedom? El Salvador, Botswana, Belize and Suriname, to list just a few.

The U.S. could again climb the rankings if it pulls back on its aggressive tack on leaks and if Congress passes a strong federal shield law, which would protect reporters' rights to not reveal their sources without fearing prosecution.

Most states have some type of shield law, but there is still no shield law to protect journalists in federal cases.